1. Field of the Invention
The present invention relates to a ceramic sheet product and a method of manufacturing the same, and more particularly, to a ceramic sheet product having a binder layer separately formed in a lower portion thereof to increase stackability and minimizing pores in a ceramic sheet after binder burnout, and a method of manufacturing the same.
2. Description of the Related Art
In the field of wireless communication, small-sized ceramic sheet products having various functions and usable in radio frequency bands are employed lately. The ceramic sheet products are manufactured by a low temperature cofired ceramic (LTCC) process which involves stacking and firing at a low temperature.
FIG. 1 is a diagram illustrating a general LTCC process.
First, a slurry 1 is prepared by mixing a ceramic powder, an organic binder, and a dispersing agent in a solvent. Then the mixed slurry 1 is applied on a plastic film 2 such as a Mylar (a trademark owned by Dupont Teijin Films) film to cast each ceramic green sheet 3, and the cast ceramic green sheet 3 is cut into a desired size.
Then, the plurality of ceramic green sheets 3 are compressed at a low pressure into one layer 4.
Next, one of a circuit pattern and an electrode is printed on the one layer 4 made up of the plurality of ceramic green sheets 3. The layers 4a to 4c each with one of the circuit pattern and the electrode printed thereon are stacked and compressed into one bar 5. The bar 5 is cut into chips 6, which are then fired at a low temperature (about 850° C.) to eliminate organic substances thereof including the binder and to densify the structure.
In the slurry prepared as described above, the binder is a highpolymer with a molecule amount of about 30, 000 to 80,000, and supports bonding between the substances to maintain a shape and mechanical strength of the green sheets. Also, in order for the binder to be uniformly dispersed among particles of ceramic powder, a process of pulverizing and mixing is applied only to the ceramic powder, the dispersing agent and the solvent for a long time in advance.
However, the high polymer binder with relatively high viscosity tends to degrade the dispersibility of substances in the slurry, eventually causing uneven dispersibility of the binder because of the ceramic powder and other substances in the solvent. This results in a non-uniform distribution of the binder in the green sheet. That is, stable rearrangement of the particles of the ceramic powder is hampered by the high polymer of high viscosity during a drying process for forming the green sheet.
In order to obtain uniform dispersibility of substances in the slurry and maintain viscosity of the slurry, the type and composition of the solvent for achieving a maximum solubility is determined according to the type of binder chosen. That is, once the binder is determined, the type of solvent is limited in selection, and therefore it is impossible to consider an optimal solvent for other substances including the dispersing agent. In other words, it is less likely to select an optimal solvent for all the substances in the slurry.
In particular, when the green sheets are stacked, inter-layer bonding is generated by the binder, and an insufficient amount of binder may result in inadequate inter-layer bonding. Conversely, too large an amount of binder may increase the number of pores after firing, which in turn decreases sinterability. Furthermore, even if a suitable amount of binder is used, spaces once occupied by the binder among the particles of ceramic powder remain as pores after firing. Therefore, the resultant structure should be densified by shrinkage during firing, but a greater number of pores makes it more difficult to accurately control shrinkage of the stacked structure.